Machine vision systems having multiple cameras are known. It is also known to simultaneously acquire images from each of a plurality of cameras in a machine vision system. Machine vision systems having a single camera for semiconductor wafer inspection, guidance, gauging, and location are also known. However, presently, it is not possible to precisely coordinate the fields of view of the plurality of cameras so that measurements can be performed across multiple fields of view to a precision of fractions of a millimeter, especially when there is significant image distortion in each field of view.
In most machine vision applications where the object of interest is a semiconductor wafer, or portion thereof, a single standard-resolution camera provides adequate resolution and scene area coverage. In such applications where higher resolution is needed, magnifying optics can be used, but scene area coverage is reduced. Alternatively, a higher-resolution camera can be used, thereby preserving scene area coverage, but such cameras can be prohibitively expensive, or unavailable at the resolution required by the application.
Resolution is defined as the number of pixels that correspond to each unit area in a physical scene. Thus, resolution determines how much scene area can be represented by each pixel of an image provided by a vision camera. A high resolution camera allocates a large number pixels to each unit area. Thus, each pixel of an image provided by a high-resolution camera represents a smaller portion of the physical scene than each pixel of an image provided by a low-resolution camera, assuming that both cameras are equipped with the same optics and are located at the same position.
In some machine vision applications, both high resolution and coverage of widely separated portions of a large scene are simultaneously required. It is clear that one solution to the problem of achieving high resolution images of a scene area greater than can be covered by a single camera is to use more than one camera, each camera having a field-of-view that covers only a portion of the scene area.
However, when there is more than one field of view, the relative physical positions of the respective reference origins of the fields of view are inherently indeterminate without reference to a common coordinate system. Consequently, position information in the image from each field of view cannot be related to position information from the other fields of view. Therefore, information interrelating the coordinate systems of multiple fields of view must somehow be provided.
Although such information can be obtained visually, by capturing in each field of view an image of a landmark disposed at a known position in relation to a common coordinate system, position information so-obtained in a machine vision system may not be sufficiently precise for many applications. This is due to lens-related and camera-related distortion effects. Unfortunately, attempting to correct these distortion effects may degrade the final accuracy of the visually acquired landmark position information relative to the common coordinate system.